1. Field of the Invention
The present invention relates to an oxygen sensor for detecting an oxygen concentration in a gas to be measured, especially an exhaust gas from an internal combustion engine.
2. Related Art Statement
Heretofore, there has been known an oxygen sensor used for detecting an oxygen concentration in an exhaust gas from an internal combustion engine and to provide for optimum control of a burning condition in the internal combustion engine in response to a detection signal thereof so as to eliminate undesirable components in the exhaust gas and result in fuel savings.
In general, such as oxygen sensor comprises an oxygen sensor element composed of a measurement electrode exposed to a gas to be measured such as an exhaust gas or the like, a standard electrode exposed to a standard substance having a constant oxygen concentration and an oxygen ion-conducting solid electrolyte body interposed between the measurement electrode and the standard electrode, said oxygen sensor generates a signal based on the oxygen ion conduction between both the electrodes as a detection signal. Usually, air is used as the above standard material. Therefore, the conventional oxygen sensor is provided at its outer casing surface with an air inlet for contacting the air with the standard electrode of the oxygen sensor element.
However, in the oxygen sensor provided with the air inlet, when it is mounted on the vehicle as described above, water or sea water might intrude into the oxygen sensor through the air inlet, which brings about a possibility that the oxygen sensor element might be damaged or might not be conductive. Moreover, since the oxygen sensor is used under high temperature conditions, water which intrudes into the oxygen sensor is changed into vapor to discharge the standard material such as air from the oxygen sensor, so that the detection of the oxygen concentration can not be performed effectively.
In order to solve the above problem, there has been proposed an oxygen sensor, wherein the air inlet is excluded and a small space is made in a porous body around the standard electrode, into which air, for example, oxygen from the exhaust gas, is supplied by means of an oxygen pump. In this oxygen sensor, since an excess gas in the small space is discharged through a slight gap formed therein to make an oxygen partial pressure in the small space constant, or since it is necessary to supply oxygen in the small space, an amount of which is equal to that consumed in reading an output signal of the oxygen sensor by means of an external instrument or the like, it is necessary to continuously operate the oxygen pump. In this case, since the oxygen pump does not operate effectively under low temperature, it is necessary to apply a high voltage to the oxygen pump electrode so as to operate it effectively under low temperature. However, if such high voltage is applied, electrolysis is liable to occur in the solid electrolyte, resulting in the deterioration of the oxygen sensor element. In order to avoid such a phenomenon, it is considered that the oxygen pump portion is always heated by means of a heater at a high temperature for operating the oxygen pump effectively. In this case, however, there are drawbacks that the amount of power consumption for the heater becomes large, the oxygen sensor element deteriorates rapidly due to heat from the heater and the service life of the heater becomes short.